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<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
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    <title>Document</title>
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<body>
    <script>
    a='12'+9;
    console.log(a,typeof a);//129 string
    b='12'-9;
    console.log(b,typeof b);//3 "number"
    // 除了 +，其他运算符运算都是数学运算
    </script>
</body>
</html>